1. Field of the Invention
The present invention relates to a lens with an optical low-pass filter incorporated therein and it also relates to a still-picture type camera or a moving-picture type video camera with such lens.
The lens according to the present invention enables the optical low-pass filter to be disposed near a lens unit that is moved along the optical axis of the lens unit. The movement of the lens unit, which is disposed between an object and the low-pass filter, enables the objective distance or the focal length of the lens to be varied, which in turn expands the potential of the design of the lens.
2. Description of the Related Art
Lens for use in a video camera or the like employs an image pickup element, such as a CCD (charge coupled device), capable of reproducing an image by discretely sampling image information received from an object. However, when the information from the object contains components with spatial frequencies exceeding the critical frequency of the image pickup element, moire fringes which do not originally belong to the object appear on the screen. Alternatively, thick stripes of local maxima and minima of density appear in a thin stripe pattern, that is, a so-called "beat disturbance" occurs.
More specifically, the frequency components of the object information which cannot be picked up by a camera cannot be reproduced as image information and this causes what is called wave-form distortion (aliasing).
In a conventional lens for use in, for example, a video camera, this aliasing is restricted by providing an optical low-pass filter in the optical path of the lens. The rays of light coming from the object pass through this optical low-pass filter, whereby they are directed into a plurality of directions. This results in the formation of the image of one spot on the image forming surface as a plurality of spots.
The high frequency characteristics of the object are thus limited so as to limit the effects of the aliasing.
Various types of optical low-pass filters are known. Examples include one which utilizes the double refraction caused by a uniaxial crystal such as rock crystal, and one which utilizes the diffraction effects of a diffraction grating disposed in the optical path of the lens.
The diffraction grating employed in a low-pass filter has a cyclic pattern that is either a, a sine curve or a trapezoidal pattern. A diffraction grating having a trapezoidal pattern is disclosed in the specifications of, for example, U.S. Pat. Nos. 3,821,795 and 3,784,734 and Japanese Patent No. sho 45-29614.
In particular, an optical low-pass filter of the type which employs a diffraction grating can be readily manufactured by molding a plastic, and such filter is therefore inexpensive. Accordingly, such filters have been widely used in lenses in recent years.
An optical low-pass filter employing a diffraction grating ensures desired low-pass effects no matter where it is disposed within the optical path of the lens. However, it is desirable to incorporate it into the lens to project it from dust and damage to the lens surface. In addition, incorporating the low-pass filter into the lens reduces the overall size of the lens unit.
However, incorporation of a diffraction grating in the lens can cause the following problems, such as those described below in detail with reference to FIG. 3.
FIG. 3 illustrates the optical function of part of an optical system in which an optical low-pass filter employing a diffraction grating is incorporated in a lens.
In FIG. 3, an optical low-pass filter 1 employing a diffraction grating is capable of separating a beam incident upon it into two bundles of rays 4a and 4b which project in different directions which form an included angle of, for example, .theta.. A movable rear lens unit 2 having a positive refracting power includes at least one movable lens element. The rear lens unit 2 is disposed on the side of the optical low-pass filter 1 closer to an image forming surface 3. The rear lens unit 2 may include another movable lens element or a fixed lens element. A solid state imaging device with a color filter is displaced on the image forming surface 3. The rear lens unit 2 receives the rays of light from the optical low-pass filter, and produces an image 1A.
Rays of light coming from an object pass through a lens unit disposed near the object (not shown) and reach the optical low-pass filter 1. The light incident upon the low-pass filter 1 emerges from the filter as diffracted light having the intensities shown in FIG. 4 due to the spatial frequency characteristics of the optical low-pass filter 1. The two diffracted bundles of rays 4a and 4b having different orders of diffraction emerge from the optical low-pass filter in two separate directions which form an included angle .theta.. The rays then pass through the rear lens unit 2 disposed on the side of the filter closer to the image forming surface, and form two spot images 5a and 5b on the image forming surface 3.
The individual spot images 5a and 5b are equivalent to those formed when the optical low-pass filter 1 having a separation angle of .theta. is disposed at the position of an image 1A formed by the rear lens unit 2 using the rays of light which pass through the optical low-pass filter 1. From this fact, it is apparent that the distance D between the two spot images 5a and 5b formed on the image forming surface 3 is expressed by EQU D=.vertline..theta.' l'.vertline. . . . (1)
where .theta.' is an extremely small angle and l' is the distance between image 1A and the image forming surface 3.
Thus, as long as the distance D between the two spot images 5a and 5b formed on the image forming surface 3 is constant, desired low-pass effects can be obtained without varying the spatial frequency characteristics of the optical low-pass filter 1.
However, when the movable lens unit in the rear lens unit 2 is moved, for example, in zooming or focusing, the position and the magnification of the image 1A formed by the rear lens unit 2 using the rays of light from the optical low-pass filter 1 vary, thus varying the position of the spot images 5a and 5b on the image forming surface 3 and, hence, the distance D between them.
This variation in the distance D varies the spatial frequency characteristics of the optical low-pass filter 1, thus prohibiting the desired low-pass effects from being obtained.
This problem may be avoided in the conventional video camera or the like by disposing the optical low-pass filter on the side of a fixed focal length lens closer to the object or immediately in front of a final lens unit of an optical system which consists of a fixed lens unit. This, however, limits the optical design of the optical system.